1. Field of the Invention
The present invention relates, in general, to driver circuits, and, more particularly, to systems and methods for compensating for back electromotive force voltage associated with the loads of switched voltage-mode power drivers and other circuits.
2. Relevant Background
Switched power driver circuits are widely used to generate power suitable for driving loads such as motors. Switched power drivers turn on and off repetitively to supply regulated voltage or current in an efficient manner (i.e., with minimal switching loss). Switched power driver circuits are associated with driver circuits that control, for example, the magnitude (by means of the duty cycle of the on and off cycles), so as to supply a desired amount of power to a load. In a typical application, a power driver circuit is controlled by a system processor, often implemented as a microcontroller IC, that generates commands to the driver circuit. The driver circuit essentially turns on and off in a predefined sequence in response to the received commands. When on, the driver circuit supplies current to the load, and when off, the driver circuit cuts off current supply to the load.
Permanent magnet motor loads, such as VCM (Voice Coil Motor), act as a motor or a generator. As a motor, the load provides a motion as a response to a voltage or current input. Additionally, if the load is in motion, it can generate a back electromotive force voltage (VBEMF). VBEMF subtracts from the applied voltage such that the motor acceleration responds to the difference in the two voltages.
In a typical application, such as a VCM motor in a disk drive, the best performance in the head positioning servo system requires that the current in the motor be proportional to the to the servo controller command. This helps both the positioning of the head over the data track as well as moving the head from the track in an efficient manner.
Head position control is implemented by a servo control system. Early servo control systems for low density drives used open loop positioning using stepper motor technology. However, at higher densities closed loop solutions are required. Current disk drives, for example, obtain head position information directly from data contained on the disk surface. A track number, in the form of encoded binary data, is recorded at various locations about the disk surface and uniquely identifies each recording track on the disk. Servo position, in the form of sinusoidal burst signals staggered in position between adjacent tracks can be used to determine the position of the head with respect to a track centerline. The track number and servo burst are used to compute a position error signal (PES), which is fed into the electromechanical servo position system.
In operation, a disk drive controller generates a command to move the head to a particular location, and the command is translated into voltage signals applied to the VCM. The voltage signals, often called drive signals, may be linear or switched-mode. Switched mode drivers are also known as pulse width modulation (PWM), phase shift modulation (PSM) and other names. Switched-mode drivers can be implemented as current-mode (i.e., use a current minor loop) or as voltage-mode drivers. These switched mode drivers have the advantage of reducing the power dissipation to the driver devices and therefore allow smaller devices and packages. In a current mode driver, a feedback loop is typically used to compare the actual current applied to the VCM to the requested current. The applied signal is compared to the command to compute a current error signal (CES). The CES is used to modify the applied signal in order to generate the requested current, so that the head eventually moves to the desired location and reduces the PES value. It is therefore desirable to have the load current proportional to the command due to the fact that the motor torque (or force in linear motors) is proportional to current. This allows for a higher performance head positioning servo system.
In the case of the driver for a VCM in a disk drive, a typical circuit topology for the feedback loop is one that is termed a xe2x80x9cCurrent Minor Loopxe2x80x9d (CML). This refers to a feedback circuit that generates a signal that is proportional to or indicative of the current magnitude in the load. The xe2x80x9cminorxe2x80x9d loop term indicates that there is localized feedback in the driver circuit in contrast to the xe2x80x9cmajorxe2x80x9d loop that controls the head position. The current in the load is sensed by an external current sense resistor, for example, that is coupled in series with the load. This resistor is typically a high precision power resistor that is relatively expensive. The back electromotive force of the motor load subtracts from the applied voltage such that the current in the motor is no longer just a function of the command but also the motor velocity. In the case of a CML, the motor current is sensed by the voltage across the current sense resistor in series with the motor and compared to the command. The voltage applied to the motor is adjusted in a closed loop manner such that the motor current remains proportional to the input command to the CML.
The voltage across the current sense resistor is brought into a control IC through an additional pin. The measured voltage is compared to the presently requested command value and a corrected command value is applied to the power drivers to obtain a load current that is proportional to command value. This extra IC pin required to port in the resistor voltage is not desirable in highly integrated circuits due to an increase in package cost. Moreover, the extra IC pin displaces other functionality that could be implemented using the pin. As higher levels of circuit integration are desired and the reduction of external components and pins is desirable due to cost and circuit area constraints, the CML topology is undesirable.
One topology that eliminates the need for the external current sense resistor and additional pin is a voltage-mode driver. Voltage mode power drivers refer to a class of control circuits that control the output voltage as opposed to output current. Voltage mode drivers are desirable because they require fewer device I/O pins and no current sense resistor. Additionally, the CML topology requires a specialized differential amplifier to process the voltage signal from the current sense resistor. This adds area to the die and development costs. Presently available switched voltage-mode power drivers do not sense the current in the load and therefore have no means to correct for the back electromotive force voltage of the load.
While voltage and current in a permanent magnet load are related, a voltage-mode driver regulates current in the motor load indirectly by regulating the applied voltage. In this case, the average voltage output of the driver is proportional to the command value. When the load is purely resistive, the output average voltage value is proportional to current. In this case (resistive load), the output voltage may be sensed without the external current sense resistor and additional pin. However, with a permanent magnet motor load, the inherent generation of back electromotive force voltage in the motor prevents the use of that topology.
Another alternative is to use a switched mode driver technique. Switched mode drivers are also known as pulse width modulation (PWM), phase shift modulation (PSM) and other names. Switched-mode drivers can be implemented as current-mode (i.e., use the current minor loop) or as voltage-mode drivers. These switched mode drivers have the advantage of reducing the power dissipation to the driver devices and therefore allow smaller devices and packages.
In a device driving a permanent magnet motor load such as a VCM, that is both switched mode and voltage mode, the resulting steady state output current to the load is directly proportional to the average voltage applied to the load terminals minus the VBEMF. (IA=(VAVExe2x88x92VBEMF)/RL), where RL is the resistance of the motor load. In a voltage mode driver, the average applied voltage VAVE is proportional to the input command. The VCM generates VBEMF in proportion to its velocity. Therefore, for a step of command input, the current in the actuator will decrease as the VCM increases velocity. It is therefore desirable to cancel the voltage loss due to VBEMF to keep the current applied to the actuator, IA, proportional to the command.
The command to the power driver is updated at a rate that is matched to the feedback information rate of the system. In a disk drive this is the servo sector rate. Since velocity can increase significantly between servo sectors, the rate that the commands are provided by the main loop may be too slow to effectively compensate for the VBEMF. It is therefore desirable that the driver correct at a much faster rate.
Briefly stated, the present invention involves a power driver for driving a signal on a load using a voltage mode driver. A system processor generates commands indicating a programmed drive signal desired for the voltage-mode driver. A compensator determines a command compensation factor to offset the back electromotive force voltage associated with the load. The compensated commands, a combination of the command compensation factor and the command, are coupled to the voltage-mode driver, such that the voltage-mode driver generates a voltage output based upon the compensated command.
In another aspect, the present invention involves providing a compensated drive command to a voltage mode driver. A voltage mode driver is coupled to a load and generates a drive signal to the load. Drive commands indicating a programmed voltage output desired from the voltage-mode driver are provided from a processor. A command compensation for a back electromotive force voltage associated with said load is determined to cancel the back electromotive force voltage associated with the load. The drive commands are modified using the command compensation to produce a compensated drive command. The compensated drive command is supplied to the voltage-mode driver. A voltage output from the voltage-mode driver is generated based upon the compensated drive command.